The spread of portable electronic equipment, such as, for example, laptop and palmtop computers, mobile telephones or video cameras, and thus also the demand for lightweight and high-performance batteries, has increased dramatically worldwide in recent years.
In view of this suddenly increased demand for batteries and the associated ecological problems, the development of rechargeable batteries with a long service life is of constantly increasing importance.
Lithium ion batteries and double layer capacitors with very high capacities (so-called super- or ultracapacitors) represent the current state of the art. In both systems, hydrolysis-sensitive and thermally unstable substances in the form of LiPF6 or N(C2H5)4BF4 are currently used as conductive salt. In contact with moist air or with residual water from the solvents, HF can form rapidly. Besides the toxic properties, HF has a very adverse effect on the cycle behavior and thus on the performance of the electrochemical cells.
Alternatives which have been presented are imides, such as bis(trifluoromethylsulfonyl)imide or bis(pentafluoroethylsulfonyl)imide, or methanides, such as tris(trifluoromethylsulfonyl)methanide and derivatives thereof. These salts exhibit high positive-electrode stability and, with organic aprotic solvents, form solutions of high conductivity. However, the imides have the disadvantage that they do not sufficiently passivate the aluminum metal which functions as negative-electrode current collector in batteries. By contrast, methanides can only be prepared and purified at very great effort (Turowsky, Seppelt, Inorg. Chem., 1988, 2135). In addition, the electrochemical properties, such as oxidation stability and passivation of aluminum, are very highly dependent on the purity of the methanide.